The September 1976 index was calculated by taking a quotient of the respective September and October indices (September/October) for pre-Corbula amurensis years 1967-1973, 1975, 1977-1978, 1980-1987 then averaging the quotients.

This mean quotient was then multiplied by the respective 1976 October index to generate the September index. The 1976 December index was calculated for each species by averaging the 1976 November and 1977 January indices.

In addition, due to the low number of stations sampled in Nov. 1969, the 1969 Nov. index is the average of the October and December 1969 index.